I've got a 2000TRD4x Tacoma. The 4wd is not activating now following an auxiliary light installation. There are two main electrical connections to the transfer case, one on the passenger side, the other on the driver side.

Which side of the transfer case has the connection for 4wd? (The other side, I assume, is for 2wd).

 

Check the fuse as that might have been blown.

Trace all lines for your aux light and make sure there's no short or direct contact w/ any other line. Replace all damaged lines accordingly (if visible).